Machinery's Handbook, 31st Edition
1438 Superplastic Forming and Diffusion Bonding involves the application of high-temperature, high-pressure argon gas to components. A hot isostatic press consists of a furnace within a gas pressure vessel. The components must be encapsulated in a sealed can to prevent the gas from entering the site of the bond. The advantages of diffusion bonding include the following: a) Limited microstructural changes. b) The ability to join dissimilar alloys. c) The ability to fabricate very complex shapes, especially using superplastic forming. d) Minimal deformation. e) A highly automated process that does not require highly skilled workers f) The ability to produce high quality joints so that neither metallurgical discontinuities nor porosity exist across the interface. g) Diffusion bonding is free from ultraviolet radiation and gas emission, so there is no direct detrimental effect on the environment and health and safety standards are maintained. Disadvantages: a) Slowness of the process. b) Protective atmosphere required. c) Expensive equipment. Electron beam diffusion bonding is a variant of diffusion bonding in which only the interface region is heated, resulting in considerable energy savings. The heating source is an electron beam that is swept over the area of the joint at such a speed that fusion of the titanium or aluminum alloy is prevented. A force is applied across the joint. As the heated area is very limited, higher forces can be used without the risk of plastic collapse of the components being bonded, resulting in a significant reduction of bonding time. Combined DB/SPF.— Diffusion bonding (DB) is often combined with superplastic form- ing (SPF) in the manufacture of complex structures in aerospace industry. This process is probably the most spectacular near-net shape process that has been developed specifically within the aerospace industry, and its industrial importance is such that it should be con- sidered separately. The process is used commercially for titanium and its alloys. The most common DB/SPF approaches are 2-sheet (hat-stiffened structures), 3-sheet (truss-core structures) and 4-sheet (rib-stiffed structures). The processing conditions for superplastic forming and diffusion bonding are similar, both requiring an elevated temperature and benefiting from the fine grain size. Therefore, these two processes have been combined into one manufacturing process known as DB/ SPF, which produces parts of greater complexity than sheet forming alone can. Fig. 30 schematically illustrates how combining diffusion bonding with superplastic forming creates a more complicated part shape in the same die. The combined DB/SPF process generally occurs in two steps: Step 1: The sheets have a stop-off material, such as boron nitride, placed on them in loca tions where no bonding is to occur in order to prevent diffusion bonding. Sheets are put down in layers, with stop-off areas into the die, heated together at an elevated temperature, and then bonded together by the use of pressure. d) Smooth surface finish requirements. e) Need for exceptional cleanliness. Step 2: In the same die, the SPF process is used to shape the outside of the laminated sheets. Pressure is applied by blowing gas between the sheets, usually in two phases. In the first phase, gas pressure is applied to cause a plastic stretching of the sheets, which eventually contact the die cavity and take a shape like that of the membrane. In the second phase, the pressure is increased to make the final shape of the part. This process generates a part that is very well bonded in the required locations.
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